In a power conversion system in which a plurality of power conversion devices are driven in parallel to supply powers to a load, there is variation among the output impedances in manufacturing of the power conversion devices even if the power conversion devices have the same specifications. Therefore, variation also occurs in load allocations at the time of load inrush, due to the variation among the output impedances.
In order to suppress occurrence of variation in load allocations as described above, there is a conventional method in which, when a plurality of power conversion devices are operated while being connected in parallel, each power conversion device adjusts, in accordance with an active component of its own output current or its own output active power, a reference phase of output voltage so as to reduce the active component of its own output current or its own output active power. It is noted that a synchronous generator also has such a droop characteristic and the droop characteristic is one of characteristics in parallel operations of a plurality of power generators in a power grid.
For example, Patent Document 1 discloses the following parallel operation system. A plurality of power conversion devices each adjust a reference phase of its own output voltage and the amplitude of output voltage in accordance with its own output current, thereby balancing output powers of the power conversion devices performing parallel operations. Further, a resistance is assumed to be provided in series between a load and a harmonic filter connected to the output end of each power conversion device, voltage drop due to the virtual resistance is calculated from the output current, and the voltage drop is subtracted from a voltage command, whereby resonance among the power conversion devices is suppressed. Thus, even if the line impedances between the load and the respective power conversion devices are different, equal allocation for a nonlinear load such as a rectifier load can be performed and each power conversion device can stably perform parallel operation.
Patent Document 2 discloses that, in a complex power generation system such as a micro-grid, each power conversion device is regarded as a power generator having an internal electromotive voltage command and an impedance, and a plurality of power conversion devices each adjust a reference phase of its own output voltage in accordance with its own output active power, and adjust the internal electromotive voltage command in accordance with its own output reactive power, thereby balancing output powers of the power conversion devices performing parallel operations. Further, the power conversion devices have current control, and when a virtual internal impedance is connected between a power supply of measured voltage and a power supply of internal electromotive voltage, the value of current flowing through the internal impedance is outputted as a command value for output current.